With recent implementation of blue LEDs, studies are being aggressively made to develop a white LED using the blue LED. The white LED requires low power consumption compared with existing white light sources and has a long life, and therefore expansion of its application to backlights for liquid crystal panels, indoor or outdoor lighting devices, and the like is proceeding.
The present white LED is obtained by coating a Ce-doped YAG (yttrium·aluminum·garnet) on the surface of blue LED. However, the fluorescence wavelength of Ce-doped YAG is in the vicinity of 530 nm and when this fluorescence color and light of blue LED are mixed to produce white light, the light is slightly blue-tinged and good white color cannot be obtained.
Meanwhile, an α-sialon-based phosphor activated by a rare earth element is known to emit fluorescence of which the wavelength is longer than the fluorescence wavelength of Ce-doped YAG (is shifted to red side) (see Kokai (Japanese Unexamined Patent Publication) No. 2002-363554). In the production method of a phosphor described in Kokai No. 2002-363554, silicon nitride is used as a raw material and after mixing a calcium source, a rare earth metal source and an aluminum source all at once, the reaction to sialon is allowed to proceed while compressing the powder by using a hot-press method.
On the other hand, Kokai No. 2005-162808 discloses a production method of a sialon-based phosphor, comprising using, as the raw material, a silicon nitride powder containing from 5 to 95 wt % of crystalline silicon nitride, mixing a calcium source, a rare earth metal source and an aluminum source all at once with the powder, and allowing the reaction to sialon to proceed by using a normal-pressure firing method.
Better white light can be obtained by combining a sialon-based phosphor produced as above with light emission of blue LED and, therefore, it is expected that a phosphor material comprising a sialon-based oxynitride is put into practical use as a novel phosphor material.
However, the phosphor having a composition disclosed in JP '554 is obtained as a firm sintered body because the reaction to sialon is allowed to proceed while compressing the powder by using a hot-press method, and for forming the sintered body into powder particles, a strong grinding treatment is necessary. Even when a grinding treatment is applied, only an aggregate of massive particles each resulting from firm fusion bonding of primary particles is obtained and this is not suited for use as a phosphor powder.
On the other hand, the sialon-based phosphor produced by the method disclosed in JP '808 can be obtained as a phosphor powder that is not largely aggregated. However, in this method, the yield of the sialon powder having a particle diameter of 2 to 20 μm is less than 55 wt %. Moreover, the amount of Eu remaining in the particle after acid treatment is small, and excessive Eu needs to be added for obtaining a high-brightness phosphor powder. The obtained sialon-based phosphor is an aggregate of massive particles each resulting from fusion bonding of primary particles, and the incident light is scattered on the aggregate surface in a large ratio due to the effect of irregularities on the outer periphery of the aggregate particle. Furthermore, there is a problem that as the particle becomes finer, the brightness of the phosphor decreases. As for the fluorescent material, a phosphor powder having a particle diameter of 2 to 20 μm, being less aggregated and causing no reduction in the emission intensity even when the powder becomes a fine particle is preferred.
This disclosure has been made to solve the above-described problems and an object of the disclosure is to provide a sialon-based oxynitride phosphor having high brightness at an emission wavelength of 560 to 620 nm. Another object of the disclosure is to provide a sialon-based oxynitride phosphor having high brightness, a desired particle size distribution and excellent miscibility with a resin and thereby achieve high brightness and stabilized color tone of an illuminating device such as white LED using blue LED as the light source or of an image display device having an excitation source such as electron beam. Such a phosphor powder is suitable for forming a thin coated film and has a property that the fluorescence is uniform and the emission intensity is high. Still another object of the disclosure is to provide a novel production method of a sialon-based oxynitride phosphor, where a sialon-based oxynitride phosphor having high brightness and excellent miscibility with a resin as described above can be obtained in a high yield.